The commercial viability of producing ethanol as a fuel source from agricultural crops has generated renewed worldwide interest due to a variety of reasons which include continued and increased dependence on limited oil supplies and the fact that ethanol production is a renewable energy source.
Alcohol fermentation production processes and particularly ethanol production processes are generally characterized as wet milling or dry milling processes. Reference is made to Bothast et al., 2005, Appl. Microbiol. Biotechnol. 67:19-25 and THE ALCOHOL TEXTBOOK, 3rd Ed (K. A. Jacques et al. Eds) 1999 Nottingham University Press, UK for a review of these processes.
In general, the wet milling process involves a series of soaking (steeping) steps to soften the cereal grain wherein soluble starch is removed followed by recovery of the germ, fiber (bran) and gluten (protein). The remaining starch is further processed by drying, chemical and/or enzyme treatments. The starch is then used for alcohol production, high fructose corn syrup or commercial pure grade starch.
In general dry grain milling involves a number of basic steps, which include: grinding, cooking, liquefaction, saccharification, fermentation and separation of liquid and solids to produce alcohol and other co-products. Generally, whole cereal, such as corn cereal, is ground to a fine particle size and then mixed with liquid in a slurry tank. The slurry is subjected to high temperatures in a jet cooker along with liquefying enzymes (e.g. alpha amylases) to solublize and hydrolyze the starch in the cereal to dextrins. The mixture is cooled down and further treated with saccharifying enzymes (e.g. glucoamylases) to produce fermentable glucose. The mash containing glucose is then fermented for approximately 24 to 120 hours in the presence of ethanol producing microorganisms. The solids in the mash are separated from the liquid phase and ethanol and useful co-products such as distillers' grains are obtained (FIG. 1A).
Improvements to the above fermentation processes have been accomplished by combining the saccharification step and fermentation step in a process referred to as simultaneous saccharification and fermentation or simultaneous saccharification, yeast propagation and fermentation. These improved fermentation processes have advantages over the previously described dry milling fermentation or even wet milling fermentation processes because significant sugar concentrations do not develop in the fermenter thereby avoiding sugar inhibition of yeast growth. In addition, bacterial growth is reduced due to lack of easily available glucose. Increased ethanol production may result by use of the simultaneous saccharification and fermentation processes.
More recently, fermentation processes have been introduced which eliminate the cooking step or which reduce the need for treating cereal grains at high temperatures. These no-cook or low temperature fermentation processes include milling of a cereal grain and combining the ground cereal grain with liquid to form a slurry which is then mixed with one or more granular starch hydrolyzing enzymes and optionally yeast to produce ethanol and other co-products (U.S. Pat. No. 4,514,496, WO 04/081193 and WO 04/080923) (FIG. 1B).
While no-cook or low temperature fermentation processes using a milled grain slurry in combination with granular starch hydrolyzing enzymes offers certain improvements over previous processes, the dry solids staging fermentation process of the instant invention provides further advantages for the production of alcohol and other end products. Some of these advantages include, but are not limited to:
a) elimination of a slurry or feed tank comprising substrates containing granular starch which feeds into a saccharification vessel;
b) decreases in the potential for microbial contamination in the fermentation of nonsterile granular starch containing substrates because of the elimination of a slurry step before the saccharification and fermentation;
c) improved mixing, faster hydration of the substrate, and improved carbon conversion efficiency because of a lower % DS in the starting mash of the initial fermentation;
d) overall high solids loading during the fermentation run;
e) an equal or higher ethanol concentration in the presence of residual starch levels which may be higher in other no-cook or low temperature fermentation processes of substrates containing granular starch, which are not subject to dry solids staging;
f) optional elimination of the yeast seed propagation tank;
g) reduced stress on yeast during the fermentation;
h) ability to handle a very fine milled substrate which will reduce the amount of residual starch, but will not result in adversely increasing the viscosity of the mash in the fermentation vessel; and
i) an increase in the enzyme to fermentable substrate ratio which enhances the hydrolysis of starch.